A conventional boiling water nuclear plant and its operating floor section will be outlined with reference to FIGS. 9 to 12.
FIG. 9 is an elevation view showing an example of the arrangement of a reactor building 100 of a conventional boiling water nuclear plant (corresponding to a cross sectional view of FIG. 10 taken along arrow IX-IX).
The example shown herein is based on a plant known as the ABWR. The reactor building 100 includes a base mat 101, a sidewall 102, and a ceiling 103. The mat 101 is made of reinforced concrete and several meters in thickness; the bottom surface thereof is substantially square in shape. The inside of the reactor building 100 is roughly partitioned into upper and lower sections, mainly by an operating floor 15. The upper space, including the operating floor 15, is referred to as operating floor area 104, while the lower space below the operating floor 15 is referred to as equipment area 105. In the case of the ABWR, an operating floor area sidewall 104a is provided closer to the inner side than an equipment area sidewall 105a. The equipment area sidewall 105a is not uniform in thickness; the lower portion is thick, while the upper portion is thin. Inside the equipment area 105, a containment vessel 3 is provided. The containment vessel 3 is cylindrical in shape (Refer to FIG. 12).
As shown in FIG. 9, a core 1 is housed in a reactor pressure vessel 2. The reactor pressure vessel 2 is housed in the containment vessel 3. The inside of the containment vessel 3 is divided into a dry well 4, which houses the reactor pressure vessel 2, and a wet well 5. The dry well 4 and the wet well 5 constitute parts of the containment vessel 3. Inside the wet well 5, a suppression pool 6 is formed. Above the suppression pool 6, a wet well gas phase 7 is formed. Outer walls of the dry well 4 and wet well 5 have been integrated to form a cylindrical outer-wall portion of the containment vessel 3 (Refer to FIG. 12). The ceiling portion of the dry well 4 is flat and is referred to as top slab 4a of the dry well 4.
In the top portion of the containment vessel 3, a containment vessel head 9, which is made of steel, is provided. The containment vessel head 9 is connected to the containment vessel 3 via flanges 10, which allow the containment vessel head 9 to be detached at the time of refueling. The outer periphery of the containment vessel head 9 is surrounded by a reactor well 11. The reactor well 11 is a space formed by a sidewall 12, which extends upwards from the containment vessel 3 in such a way as to surround the containment vessel head 9, and a base 13, which is connected to the lower end of the sidewall 12 and supports the sidewall 12. In the case of a reinforced concrete containment vessel (RCCV), the base 13 constitutes part of the containment vessel 3. In the case of a steel containment vessel, the base 13 constitutes part of shield concrete that surrounds the steel containment vessel. Typically, the horizontal cross section of the reactor well 11 is circular. Alternatively, the horizontal cross section of the reactor well 11 may be elliptical or polygonal. The sidewall 12 and the base 13 are robust structures made of reinforced concrete and are two meters or more in thickness. On the inner surface of the reactor well 11, steel liners (not shown) have been lined to ensure leak tightness.
As shown in FIG. 9, a hollow cylindrical pedestal 61 supports the reactor pressure vessel 2 through RPV (Reactor Pressure Vessel) skirt 62 and RPV support 63. As for the pedestal 61, there are various structures, such as those made of steel or concrete, or steel concrete composite. The inner space of the pedestal 61, or space of the dry well 4 that is below the reactor pressure vessel 2 and surrounded by the cylindrical wall of the pedestal 61, is referred to as a pedestal cavity 64. In the case of RCCV of the ABWR, as the cylindrical wall of the pedestal 61 constitutes the boundary wall between the wet well 5 and the dry well 4, this space is specifically referred to as a lower dry well 65. In the case of RCCV of the ABWR, the upper space of the dry well 4, excluding the lower dry well 65, is referred to as an upper dry well 66.
The dry well 4 and the suppression pool 6 are connected via LOCA (Loss of Coolant Accident) vent pipes 8. For example, although the number of the LOCA vent pipes 8 to be installed is 10 (Refer to FIG. 12), FIGS. 9 and 11 are cross-sectional and showing only two of them. The LOCA vent pipes 8 have horizontal vent pipes 8a that are submerged in the pool water of the suppression pool 6. The LOCA vent pipes 8 are opened in the pool water. In the case of RCCV, each LOCA vent pipe 8 has three horizontal vent pipes 8a, which are arranged in the vertical direction. In the case of RCCV, the LOCA vent pipes 8 are installed in such a way as to pass through the cylindrical wall of the pedestal 61. Accordingly, in the case of RCCV, the cylindrical wall of the pedestal 61 is also referred to as a vent wall. The vent wall is made of reinforced concrete and is about 1.7 meter in thickness; the inner and outer surfaces of the vent wall are made of steel. The LOCA vent pipe 8 and the pedestal 61 constitute part of the containment vessel 3.
As shown in FIG. 9, an openable equipment hatch 35 is provided to enable equipment to be moved in and out of the containment vessel 3. The equipment hatch 35 has the same levels of pressure resistance and leak tightness as the containment vessel 3 when the equipment hatch 35 is closed. Moreover, an air lock 36 is provided to enable operators to get in and out of the containment vessel 3. Usually the air lock 36 has two doors with an interlock, which prevents the doors from being opened simultaneously. The air lock 36 has the same levels of pressure resistance and leak tightness as the containment vessel 3. FIG. 9 only shows the equipment hatch 35 and the air lock 36 being installed in the upper dry well 66. However, the equipment hatches 35 and the air locks 36 are also provided in the wet well 5 and the lower dry well 65.
The containment vessel 3 has typical varieties according to its materials such as steel containment vessel, reinforced concrete containment vessel (RCCV), pre-stressed concrete containment vessel (PCCV), steel concrete composite (SC composite) containment vessel (SCCV), and so on. In the case of RCCV or PCCV, steel liners have been lined on the inner surface. FIGS. 9 and 11 show an example of RCCV. While FIGS. 9 and 11 are elevation views, the outer-wall portion of the RCCV is cylindrical (Refer to FIG. 12).
In the case of a boiling water reactor, the atmosphere of the containment vessel 3 is inerted with nitrogen during normal operation so that the oxygen concentration is kept low.
FIG. 10 is a plan view showing the arrangement of parts in the vicinity of the operating floor area 104 of the reactor building 100 of the conventional boiling water nuclear plant. In the operating floor area 104, the reactor well 11, a fuel pool 20, and a dryer and separator pit 30 are provided. FIG. 11 is an elevation view showing an example of the arrangement of the reactor building 100 of the conventional boiling water nuclear plant (corresponding to a cross-sectional view of FIG. 10 taken along arrow XI-XI). What is depicted in the figure is the same as in FIG. 9 but is turned 90 degrees. As shown in FIG. 11, an operating floor area sidewall 104b is on the same plane as an equipment area sidewall 105b. The reactor well 11, the fuel pool 20, and the dryer and separator pit 30 are provided in such a way as to extend downwards from the operating floor 15.
During normal operation, a shield plug (not shown) is placed above the reactor well 11. The shield plug blocks radiation, which is generated when the reactor is operating. The shield plug can be removed at the time of refueling.
As shown in FIGS. 10 and 11, the fuel pool 20 is provided in the operating floor area 104. A sidewall 21 that surrounds the fuel pool 20 is made of reinforced concrete and is about 2 meters in thickness. A floor 22 of the fuel pool 20 is also made of reinforced concrete, and is about 2.4 meters in thickness. The floor 22 is formed into a stepwise pattern, and part of the floor 22 is commonly used as the top slab 4a of the containment vessel 3. On the inner surface of the fuel pool 20, a steel liner (not shown) has been lined to ensure leak tightness. In this manner, the structure of the fuel pool 20 is robust and leak tightness is ensured. The fuel pool 20 communicates with the reactor well 11 via a fuel pool slot 23. During normal operation, the fuel pool slot 23 is closed with a leak-tight slot plug 24.
In the operating floor area 104, the dryer and separator pit 30 is provided on the opposite side of the reactor well 11 from the fuel pool 20. The dryer and separator pit 30 is a pool that is used to temporarily store a dryer and a moisture separator (not shown) after the dryer and the moisture separator are removed from inside the reactor pressure vessel 2 in refueling. The sidewall 31 surrounding the dryer and separator pit 30 is made of reinforced concrete and 2 meters or more in thickness. A floor 32 of the dryer and separator pit 30 is made of reinforced concrete and 2 meters or more in thickness; part of the floor 32 is commonly used as the top slab 4a of the containment vessel 3. On the inner surface of the dryer and separator pit 30, steel liners (not shown) have been lined to ensure leak tightness. In this manner, the structure of the dryer and separator pit 30 is robust and leak tightness is ensured. The dryer and separator pit 30 communicates with the reactor well 11 via a gate 33. During normal operation, the gate 33 is closed by a removable panel 34.
As shown in FIG. 10, on the operating floor 15, an equipment hatch 106 is provided. The equipment hatch 106 has a fall-prevention cover. When the equipment hatch 106 is opened, an opening 107 is formed in the operating floor 15 to allow equipment to be moved in or out. The opening 107 leads to a ground-level equipment access lock 109 through a shaft 108. The shaft 108 extends to each floor 110 inside the reactor building 100, as shown in FIG. 13. FIG. 13 is an elevation view showing the cross section of the reactor building 100 at the position where the equipment hatch 106 is located. The operation floor area 104 communicates with each floor 110 inside the reactor building 100 through the shaft 108. The equipment hatch 106 is not leak tight and allows air flow. Even if the equipment hatch 106 is closed, the operation floor area 104 therefore communicates with each floor 110 inside the equipment area 105 through the shaft 108.
As shown in FIG. 10, in the operating floor area 104, elevators 111 and staircases 112 are provided. The elevators 111 can go down to the lowest basement level of the reactor building 100 through elevator shafts 111a. The staircases 112 also lead to the lowest basement level of the reactor building 100. FIG. 13 is an elevation view showing the situation. The operating floor area 104 communicates with the equipment area 105 inside the reactor building 100 in multiple locations as offered by the elevator shafts 111a and the staircases 112.
As shown in FIG. 11, from the reactor pressure vessel 2, a main steam line 71 extends out of the dry well 4 and further penetrates the side wall 102 of the reactor building 100. A section of the main steam line 71 between the dry well 4 and the side wall 102 of the reactor building 100 is housed in the reactor building 100. On the main steam line 71, main steam line isolation valves 71a and 71b are provided; the main steam line isolation valve 71a is inside the dry well 4, and the main steam line isolation valve 71b outside the dry well 4. Besides the main steam line 71, on a penetration line 37 of the containment vessel 3, as a general rule containment vessel isolation valves 38a and 38b are provided inside and outside the containment vessel 3. The containment vessel isolation valves 38a and 38b may be motor-operated valves, air-operated valves, check valves, or the like, and have pressure resistance and leakage protection functions.
As shown in FIGS. 9 and 11, on the operating floor area sidewall 104a, blowout panels 113 are provided. If the main steam line 71 breaks outside of the containment vessel 3 but inside the reactor building 100, a large amount of steam is released into the reactor building 100. With the aim of discharging the steam out of the reactor building 100 in a controlled manner, the blowout panels 113 are provided on the operating floor area sidewall 104a. The steam rapidly reaches the operating floor area 104 via the shaft 108 of the equipment hatch 106, the elevator shafts 111a, and the staircases 112 (Refer to FIG. 13). The steam promptly pushes open the blowout panels 113 and blowout into the environment. The blowout panels 113 are designed to open at the set differential pressure of about 2 psid (or about 13.8 kPa). Therefore, the blowout panels 113 open by a slight rise in pressure in the operating floor area 104.
In the equipment area 105, important safety equipment is placed. Therefore, the equipment area sidewalls 105a and 105b, which are made of reinforced concrete and about 1 to 1.5 meters in thickness, are robust. The operating floor area sidewalls 104a and 104b are about 0.3 meter in thickness. The thickness of an operating floor area ceiling 104c is about 0.3 meter. The reactor building 100, including the operating floor area 104, is seismically designed to withstand a large earthquake, and robust, but pressure resistance is limited for internal pressurization over the set point of the blowout panels 113.
An example of an operation method of a conventional boiling water nuclear plant in refueling will be explained with reference to FIG. 14. For refueling, first the reactor is shut down, and the water level inside the reactor is raised to the level of the flange 2a of the reactor pressure vessel 2. From the reactor well 11, the shield plug (not shown) is removed. Furthermore, the containment vessel head 9 (Refer to FIGS. 9 and 11) is removed at the position of the flange 10. Then, the reactor pressure vessel head 2a (Refer to FIGS. 9 and 11) is removed at the position of the flange 2a. After that, the water level inside the reactor pressure vessel 2 is raised so that the reactor well 11 is filled.
The gate 33 of the dryer and separator pit 30 (Refer to FIG. 10) is opened in order to fill the dryer and separator pit 30 with water. Then, the dryer is detached and transferred to the dryer and separator pit 30. Then, the separator is removed and transferred to the dryer and separator pit 30. The slot plug 24 (Refer to FIG. 10) is removed so that the fuel pool 20 communicates with the reactor well 11. Then, the spent fuel is moved from the core 1 to the fuel pool 20, and new fuel is loaded into the core 1.
FIG. 14 shows part of a series of steps described above, or the situation where the reactor pressure vessel head 2b (Refer to FIGS. 9 and 11) has just been removed with the water level inside the reactor pressure vessel 2 at the flange 2a level.
Besides storing spent fuel that has been generated during normal operation, the fuel pool 20 also has a mission to temporarily store the core fuel when it is necessary to take out the core fuel due to repair work inside the reactor pressure vessel 2.
A conventional filtered venting system will be explained with reference to FIG. 15. A filtered venting system 50 has been adopted at nuclear plants in Europe since the accident at the Chernobyl nuclear plant. A growing number of plants in Japan have been employing the system since the accident at the Fukushima Daiichi nuclear plant.
FIG. 15 is an elevation view showing an example of how a conventional filtered venting system has been designed. The filtered venting system 50 includes: a filtered venting tank 51, which stores scrubbing water 52; an inlet pipe 53, which leads the gas inside the containment vessel 3 to the scrubbing water 52; and an exhaust pipe 54, which releases the gas in the gas phase of the filtered venting tank 51 to the environment.
The installation places of the filtered venting tank 51 and the like are not limited to the inside of the building. When the filtered venting tank 51 and the like are installed at an existing reactor, the filtered venting tank 51 and the like are placed outside of the reactor building 100 in many cases. Meanwhile, if the filtered venting tank 51 and the like are installed during construction, the filtered venting tank 51 and the like may be placed inside the reactor building 100 or the like.
There is a type in which a Venturi scrubber 55 is placed under the scrubbing water 52 and the gas is led from the inlet pipe 53 into the Venturi scrubber 55. However, the Venturi scrubber 55 is not necessarily required. There is also a type in which a metal fiber filter 56 is placed in the gas phase of the filtered venting tank 51. However, the metal fiber filter 56 is not necessarily required.
FIG. 15 shows the case where both the Venturi scrubber 55 and the metal fiber filter 56 are provided. On the inlet pipe 53, as one example, an isolation valve 57 is provided, and a rupture disk 58 is provided in parallel. Moreover, isolation valves 59a and 59b, which are normally open, are placed before and after the rupture disk 58.
Moreover, an exhaust valve 60 is placed on the exhaust pipe 54. However, the exhaust valve 60 is not necessarily required. In many cases, a rupture disk is used instead of the motor-operated valve. In the conventional filtered venting system, one end of the inlet pipe 53 is directly connected to the containment vessel 3 in order to take in the gas inside the containment vessel 3.
The conventional reactor building 100 lacks pressure resistance, and the blowout panels 113 could be mistakenly opened due to vibration resulting from earthquakes or the detonation of hydrogen. Moreover, when hydrogen is generated in the operating floor area 104 at the time of a severe accident, the hydrogen needs to be proactively released into the environment in order to prevent the detonation. The hydrogen generated at the time of a severe accident contains radioactive materials. The release of such hydrogen raises the risk of causing exposure and land contamination.
At the time of refueling, both the reactor pressure vessel head 2b and the containment vessel head 9 are removed. If an earthquake and tsunami causes a long-term station blackout (SBO) and then a meltdown, hydrogen and a large amount of radioactive materials would be directly released into the operating floor area 104. In this case, there are concerns that the hydrogen and radioactive materials could be released into the environment as the blowout panels 113 are opened. If the transfer of core fuel into the fuel pool 20 has been already completed, a failure to cool the fuel pool 20 could raise the risk of radioactive materials being released from damaged fuel into the environment via the blowout panels 113. Even if the filtered venting system 50 has been installed in the containment vessel 3, radioactive materials would be directly released into the environment from the operating floor area 104 via the blowout panels 113 at the time of refueling. Therefore, there is a possibility that the filtered venting system 50 could be bypassed and fail to function.
Accordingly, it is important to prevent the detonation of hydrogen and the release of large quantities of radioactive materials into the environment even when large amounts of hydrogen and radioactive materials are released into the operating floor area at the time of refueling. Moreover, even if a long-term station blackout is initiated at the time of refueling, it is important to safely cool both the core fuel and the spent fuel.
The object of embodiments of the present invention is therefore to prevent the leakage of radioactive materials into the environment even as the radioactive materials are released from the core fuel or the spent fuel.